1. Field of the Invention
The present invention relates to anisotropic phosphor deposition technology, and more particularly to a method and apparatus for manufacturing an auto-collimating phosphor imaging screen.
2. Brief Description of the Prior Art
In U.S. Pat. No. 4,069,355 to Lubowski et al., there is described a process for forming a phosphor screen on a patterned substrate using wide-angled vapor deposition (as in a hot-wall evaporator) so as to deposit the phosphor only on the raised portions of the substrate.
In U.S. Pat. No. 4,528,210 to Noji et al., there is described a process for sequentially depositing a multilayer input phosphor screen on a substantially smooth substrate so that individual columnar crystals of the second layer of alkali halide grow vertically upon the crystal particles of the first layer, standing close together with fine spaces therebetween[sic]. A third layer is preferably deposited on the second layer as a continuous film. These three layers can be deposited by evaporating a phosphor material or materials at a prescribed temperature and degree of vacuum.
In U.S. Pat. No. 4,842,894 to Ligtenberg et al., there is described a method of manufacturing an x-ray image intensifier tube wherein a vapor deposition crucible is provided between forty and fifty degrees from the central normal to a smooth surface of a screen on which luminescent material is to be deposited. During deposition the vapor deposition source performs a circular movement about the surface to produce a layer having a regular structure and a good fill factor.
In U.S. Pat. No. 5,171,996 to Perez-Mendez, there is described a method for fabricating a particle detector of a sequence of columns of regular, controllable geometry and diameter perpendicular to the interface of luminescent material with adjacent materials wherein the columns are separated by gaps which may be evacuated or filled with air, with a light-absorbing material, or with a light-producing or light-reflective substance.
In U.S. Pat. No. 5,427,817 to Goodman et al., there is described a process for the vapor deposition of a scintillator phosphor composition with concomitant shadowing wherein the substrate is rotated through an arc relative to the vapor source of said phosphor, whereby shadowing introduces voided gaps or interstices between columns as a result of the preferential components receiving more coating flux, particularly in the presence of an oblique flux. In the present invention the limitations of the past due to the mandrel substrate fixture, in the size and quantity of applicable substrates, have been eliminated by a system scale-up that required the development of a novel, computer controlled, multiconstituent, evaporation source. Furthermore, the precise monitoring and control over material feed-rates and multiple temperature zones achieved with this source provide the means to produce more complex phosphors, as will be described in the following objects and detailed description of the invention.
While the processes of the prior art have achieved certain levels of performance, there is a desire to optimize the performance of structured, auto-collimating phosphors to achieve high ionizing radiation to quantum energy conversion efficiency and high spatial resolution.
Conventional thin film deposition techniques involve changing of the surface properties of substrate materials to impart advantageous characteristics, e.g., a corrosion-resistant coating. Successful surface modification depends on the deposition of a continuous, defect-free coating layer. Conversely, an effective auto-collimating phosphor layer must be replete with uniform structural discontinuities.
An object of the present invention is to provide a process for forming an anisotropic, auto-collimating phosphor of high ionizing radiation to luminescent energy conversion efficiency with concomitant high spatial resolution.
Another object of the present invention is to provide a method for depositing an optical wave-guiding structured layer, consisting of columns and interstices of controllable geometry, to optically decouple the luminescent columns while retaining a high fill factor.
A further object of the present invention is to provide a well controlled, hence reproducible, economical method for producing a structured, auto-collimating phosphor with enhanced detector spatial resolution and energy conversion efficiency.
A further object of the present invention is to provide a physical vapor evaporation source that is able to control the evaporation rates of the constituents of complex phosphor compositions, while maintaining fixed ratios of those constituents in the vapor phase and resultant condensed phosphor layer.
A further object of the present invention is the ability of said source to evaporate very large amounts of material without reloading, which is ideal for load-locked production systems.
A further object of the present invention is the achievement of very high deposition rates for long periods of time, which is necessary for depositing thick layers of phosphors over large area substrates.
A further object is for said source to work in an inert environment, from high vacuum to many torr of pressure, because high operating pressure results in the preferential columnar deposition profile necessary to achieve the auto-collimating function.
A still further object of the present invention is to provide a method of depositing phosphor layers directly upon passivated photodetectors such as charge coupled devices, photodiode arrays, etc., by means and under conditions that do not damage the photodetectors.
A still further object of the present invention is to provide a method of depositing phosphor layers upon fiber optic elements such as face plates and light pipes so that excellent optical coupling is achieved without the occurrence of visible artifacts such as moire interference patterns.
Still another object of the present invention is to provide a method of depositing phosphor layers on supportive substrates to produce image intensifying and storage phosphor screens.
A physical vapor deposition system consisting of a suitable vacuum chamber fitted with the instrumentation, process sensors and metrology necessary to effect the deposition of a layer of phosphor complex with controlled anisotropic structure and dopant profile.